The present invention relates generally to polyolefin foams and, more particularly, to an improved blowing agent for producing such foams.
Polyolefin foams are commonly made by an extrusion process that is well known in the art. In such a process, polyolefin resin is added to an extruder, typically in the form of pellets. Common polyolefins employed in such a process include polyethylenes, such as low density polyethylene (LDPE), and polypropylene. In the extruder, the resin pellets are melted and mixed, and a blowing agent is admixed with the melted polyolefin. The blowing agent must be at least partially miscible in the polymer at the temperature and pressure conditions within the extruder such that at least a portion of the blowing agent dissolves into the molten polymer. The extruder pushes the melt mixture (melted polyolefin and blowing agent) through a die at the end of the extruder and into a region of reduced temperature and pressure (relative to the temperature and pressure within the extruder). Typically, the region of reduced temperature and pressure is the ambient atmosphere. The sudden reduction in pressure causes that portion of the blowing agent that is dissolved in the polymer to come out of solution, nucleate, and vaporize/expand into a plurality of cells within the polymer that solidify upon cooling of the polymer mass (due to the reduction in temperature), thereby trapping the blowing agent within the cells.
Following the extrusion and cooling of the foam, a concentration gradient is established between the cells of the foam and the ambient atmosphere, such that the blowing agent will have a tendency to diffuse out of the foam cells while air will have a tendency to diffuse into the cells. A problem frequently encountered with certain blowing agents is that, during this aging or curing period, the blowing agent diffuses out of the foam cells more rapidly than air diffuses into the cells, resulting in the partial or complete collapse of the cell walls and consequent reduction in the volume of the foam. This reduction in volume is highly undesirable, in that it adversely impacts the cushioning performance of the foam. The ability of a foam to resist such loss in volume is generally referred to as the "dimensional stability" of the foam.
In the past, chlorofluorcarbons, such as 1,2-dichlorotetrafluoroethane, dichlorodifluoromethane, trichloromonofluoromethane, etc., were used as blowing agents in the production of polyolefin foams because, among other reasons, the rate of diffusion of such materials out of the foam was slow enough to approximate the rate of diffusion of air into the foam to prevent significant cell wall collapse. Relative to other potential blowing agents, however, chlorofluorcarbons are expensive and believed to be harmful to the environment. The continued use of such materials as blowing agents, therefore, is highly undesirable.
Accordingly, efforts have been made to use less expensive and more environmentally-friendly blowing agents that also provide good dimensional stability to polyolefin foams. For example, U.S. Pat. Nos. 4,694,027, 4,640,933, and 4,663,361 disclose, as a complete or partial replacement for chlorofluorcarbon blowing agents, a mixture of at least 70% isobutane with a physical blowing agent selected from the group consisting of hydrocarbons, chlorocarbons, and chlorofluorocarbons having from 1 to 5 carbon atoms, boiling points between -50.degree. C. and 50.degree. C., and a permeation rate through an olefin polymer resin modified with a stability control agent of greater than about 1.2 times the permeation rate of air.
While such blowing agents composed primarily of isobutane have proven to be adequate replacements for chlorofluorcarbons in many respects, a number of drawbacks have been identified. First, the permeation rate of isobutane through polyolefins is higher than would ideally be desired, resulting in lower-than-desired dimensional stability in polyolefin foams made from blowing agents having a high isobutane content (i.e., greater than about 65%). In addition, the solubility of isobutane in polyolefins, particularly in polyethylenes such as LDPE, is low in comparison to other available blowing agents such that it is difficult to produce lower density polyethylene foams, i.e., 1.5 pounds/ft.sup.3 or less, at foam `plank` thicknesses of about 1.5 inches or more when the blowing agent used has a high isobutane content because less dissolved blowing agent is available for vaporization and expansion into cells within the polymer. Also for this reason, very low foam densities of 1 pound/ft.sup.3 or less are difficult to attain with blowing agents having a high isobutane content at any foam thickness, e.g., even at foam `sheet` thicknesses of 1 inch or less.
A further drawback of high-isobutane-content blowing agents is that isobutane has a relatively high degree of volatility, which makes it difficult to produce foam planks having a thickness of greater than about 1.5 inches, regardless of the foam density. This is because a larger die opening is required to produce a thicker foam. A larger die opening, however, generally results in less pressure within the die, which can lead to premature vaporization of the blowing agent, and therefore cell formation, inside the die when higher volatility gases such as isobutane are employed as major components of the blowing agent. Such premature foaming is undesirable because it produces cell coalescence and other irregularities in cell structure, both of which deleteriously affect the quality and cushioning properties of the foam.
Accordingly, a need still exists in the art for an improved alternative to chlorofluorcarbon blowing agent for use in the production of polyolefin foams.